1. Field of the Invention
The present invention relates to development of a quaternary metal catalyst based on platinum (Pt)-ruthenium (Ru) for a direct methanol fuel cell (DMFC), that is, an anode catalyst, which is essential materials determining the performance of a DMFC. More particularly, the present invention relates to a quaternary metallic anode catalyst for a DMFC, consisting of platinum (Pt), ruthenium (Ru), M1 and M2, the M1 and M2 being selected among transition metals from Groups V-XI of the Periodic Table of the Elements, respectively.
2. Description of the Related Art
Recently, as portable electronic devices and cordless communication equipments have been rapidly developed, much attention is being paid to development of fuel cells as portable power sources, fuel cells for pollution-free automobiles and power-generation systems as clean energy sources.
A fuel cell is a new power generation system for directly converting the chemical energy of fuel (gas), e.g., hydrogen or methanol, and an oxidizer, e.g., oxygen or air, into electrical energy. There are several different fuel cells: molten carbonate fuel cells operating at higher temperatures of approximately 500 to approximately 700° C.; phosphoric acid fuel cells operating at approximately 200° C.; alkaline electrolyte fuel cells and polymer electrolyte fuel cells operating at below 100° C. or at room temperature.
The polymer electrolyte fuel cell is subdivided into a proton exchange membrane fuel cells (PEMFC) using hydrogen gas and a direct methanol fuel cell (DMFC) using liquid methanol, according to anode fuel. The polymer electrolyte fuel cell, which is a source of future clean energy that can replace fossil energy, has high power density and high energy conversion efficiency. Also, the polymer electrolyte fuel cell can operate at room temperature and can be made miniaturized. Thus, the polymer electrolyte fuel cell has very wide applications including zero-emission vehicles, home power generation systems, and power source for mobile communications equipment, medical appliances and military equipment for example.
In general, a proton exchange membrane fuel cell using hydrogen is advantageous in that it has high power density, but cautious handling of hydrogen gas is needed and there is demand for an additional facility, such as a fuel reforming apparatus for reforming methane, methanol or natural gas to produce hydrogen fuel.
On the other hand, although having lower power density than gaseous fuel cells, a direct methanol fuel cell is considered to be suitable as a small and general-purpose portable power source from the viewpoints of manageability, low operation temperatures and lack of necessity of additional fuel reforming apparatus.
Referring to FIG. 1, a fuel cell is constructed such that a proton exchange membrane 11 is interposed between an anode and a cathode. The proton exchange membrane 11 has a thickness of 50 to 200 μm and is made of solid polymer electrolyte. Each of the anode and cathode of such a cell includes a gas diffusion electrode consisting of a support layer 14, 15 for supply and diffusion of each reactant gas and a catalyst layer 12, 13 at which oxidation/reduction of the reactant gas occur (the anode and the cathode may also be collectively termed a gas diffusion electrode), and a current collector 16.
In the anode of a DMFC, methanol oxidation occurs to produce protons and electrons. The produced protons and electrons are transferred to the cathode. In the cathode, the protons react with oxygen, that is, reduction occurs. An electromotive force based on electrons from anode to cathode is an energy source of a fuel cell. The following reaction equations represent reactions occurring in the anode and cathode and an overall reaction occurring in the single cell.
[Anode (Negative Electrode)]
 CH3OH+H2O→CO2+6H++6e− Ea=0.04 V
[Cathode (Positive Electrode)]3/2O2+6H++6e−→3H2O Ec=1.23 V
[Single Cell]CH3OH+3/2O2→CO2+2H2O Ecell=1.19 V
The overall performance of a fuel cell is greatly influenced by the performance of anode materials because the anode reaction rate is slow. Thus, in order to realize commercialization of DMFCs, development of superb catalysts for methanol oxidation is quite important.
While the methanol is electroadsorbed onto the platinum surface and oxidized to make protons and electrons, the catalyst poison which is linearly bonded carbon monoxide on Pt to make Pt useless is produced.
It was reported that resistance to catalyst poison in a platinum (Pt) catalyst could be enhanced by combining ruthenium (Ru) with Pt, preferably in the atomic ratio of 50:50 (D. Chu and S. Gillman, J. Electrochem. Soc. 1996, 143, 1685). The enhanced resistance to carbon monoxide is based on the ability of Ru adsorbing H2O molecules at chemical potentials where methanol is adsorbed on Pt. This bifunctional mechanism explains the promotion of catalyst activity by interaction of transition metals and is represented by the following reaction.Pt—CO+Ru—OH→Pt+Ru+CO2+6H++e−
In DMFCs, anode materials are being developed predominantly using a Pt—Ru binary alloy catalyst, which is already partially commercialized. Thus, much research is focusing on Pt based binary anode catalysts such as Pt—Mo, Pt—W, Pt—Sn or Pt—Os, and Pt—Ru based ternary catalysts such as Pt—Ru—Os or Pt—Ru—Ni. However, the use of the binary catalysts or ternary catalysts makes it difficult to obtain both a bifunctional effect and an electronic effect. Therefore, the present invention proposes quaternary metallic catalysts which are effectively used for methanol oxidation. Actually, the quaternary metallic catalysts have been difficult to be practically used because they have difficulty in alloying between metals. Both the bifunctional effect and the electronic effect can be obtained from the use of the quaternary metallic catalysts, making it possible to increase the catalyst activity effectively. Although it is not easy to develop quaternary metallic catalysts due to the difficulty of metallic solubility and the catalyst design, the quaternary metallic catalysts are being expected to exhibit good catalyst activity compared with existing catalysts. The phase equilibrium, atomic bonding strength and knowledge of catalyst activity are vital parameters in selecting elements and determining combination ratios of such elements. The inventors carried out experiments on various transition metals for attainment of new anode catalysts having better performance than conventional anode catalysts. The experiments have ascertained that the quaternary anode catalysts according to the present invention promote CO oxidation by adding easily-OH-adsorbing metals and weaken CO—Pt bond by changing the electronic structure between atoms, thereby mitigating CO poisoning to thus increase catalyst activity, and improving performance of fuel cells.